Two spool turbofan and turboprop jet engines typically include three sections in the core. The first section is the compressor section, which includes a low pressure compressor (LPC) followed by a high pressure compressor (HPC). A combustor section is disposed between the compressor section and a turbine section, which includes a high pressure turbine (HPT) followed by a low pressure turbine (LPT). The LPT is connected to and drives the LPC via one shaft and the HPT is connected to the HPC via a second shaft.
Turbofan and turboprop engines operate by combusting fuel in air that has been compressed by the LPC and HPC of the compressor section. The combustion takes place in the combustor section to create heated gases with increased pressure and density. The heated gases are used to rotate the HPT and LPT in the turbine section that are used to produce thrust or power. For example, in a turbofan, the heated gases are ultimately forced through an exhaust nozzle at a velocity higher than which inlet air is received into the engine to produce thrust for driving an aircraft. The heated gases also rotate the HPT and LPT, which are used to drive the HPC and LPC respectively, which generate the compressed air necessary to sustain the combustion process.
The compressor (LPC and HPC) and turbine (HPT and LPT) sections of a turbofan or turboprop engine typically comprise a series of stator vane and rotor blade stages. The stator vanes of each stage are positioned in front of a rotor to efficiently direct air flow to the blades of the rotor. In general, the stator vanes redirect the trajectory of the air coming off the rotors of the preceding stage for flow into the next stage.
In the compressors, the stator vanes convert kinetic energy of the moving air into pressure, while, in the turbines, the stator vanes accelerate pressurized air to extract kinetic energy. Turbofan and turboprop efficiencies are, therefore, closely linked to the ability of the engine to efficiently direct air flow within the compressor and turbine sections of the engine. Air flows through the compressor and turbine sections differ at various operating conditions of the engine, with more air flow being required at higher output levels and vice versa.
Variable stator vane assemblies are utilized to improve the performance and operability of the engine. Variable stator vane assemblies typically include variable stator vanes which extend forward of rotor blades. The variable stator vanes are rotatable about substantially radial axes. The orientation of the variable stator vanes varies the attack angle of the vanes in a controlled fashion. This allows the vanes to be realigned to change the impingement angle of compressed air onto the following rotor blades as the operating condition of the engine changes. The position of the vanes may be changed by many different means, including, but not limited to a lever arm attached to an actuator ring on the outside of the compressor case or a gear driven arrangement. Thus, air flow through the engine can be controlled, in part, by using variable stator vanes and variable stator vanes have been used to advantageously control the incidence of air flow onto rotor blades of subsequent compressor stages under different operating conditions.
However, schemes for controlling variable stator vanes are generally lacking. Engines without LPC variable stator vanes modify the stability of the LPC using bleed air, which detracts from the performance and efficiency of the engine.
Therefore, there is a need for a control scheme for altering the positions of the LPC stator vanes that is more flexible than the currently available control schemes and which can be used to advantageously control various operating parameters including overall compressor pressure ratio (i.e., the ratio of the pressure at the aft end of the HPC to the pressure at the forward end of the LPC), compressor corrected air flow, bypass ratio (i.e., the ratio of the air entering the core shroud to the air entering the inlet shroud), engine temperatures, spool speed (rpm), and compressor operating line, while reducing fuel consumption.